1. Field of the Invention
The present invention relates generally to the field of optoelectronics and, more particularly, to an improved method and device for providing precise optical alignment between an optoelectronic device and an optical fiber.
2. Description of the Related Art
Optoelectronics (or photonics) is a rapidly expanding technology that plays an increasingly important role in many aspects of modern society (e.g., communication over optical fibers, computer storage and display, etc.). With the increasing number of actual and potential commercial applications for optoelectronic systems, there is a need to develop cost effective and precise manufacturing techniques for assembling optoelectronic modules (e.g., fiber-optic cable repeaters and transmitters).
One of the problems associated with developing such cost effective manufacturing techniques is the criticality that the components (e.g., lasers, photodiodes and optical fibers) of such optoelectronic modules be assembled with high precision to assure proper optical coupling and durability. Typically, an optoelectronic module includes a package or housing containing an optoelectronic device (e.g., semiconductor laser, LED or photodiode) coupled to an optical fiber (e.g., single mode, multimode or polarization maintaining) that extends from the package. A major challenge in assembling such optoelectronic modules is in maintaining optimal alignment of the optoelectronic device with the optical fiber to maximize the optical radiation (e.g., light) transmitted through the optical fiber. In order to obtain maximum optical coupling, it is typically desired that the core-center of the optical fiber be precisely aligned with that of the optoelectronic device. In some cases, such as with a single-mode optical fiber, the alignment between the optoelectronic device (i.e, laser) and optical fiber must be within tolerances of 1 .mu.m or less.
A conventional method for aligning an optoelectronic laser with an optical fiber is known as "active alignment", where the laser is bonded to a substrate and one end of a desired type of optical fiber is positioned in close proximity to a light-emitting surface of the laser in order to transmit light emitted from the laser through the optical fiber. A photodetector, such as a large area photodetector, is positioned at the opposing end of the fiber to collect and detect the amount of light (optical radiation) coupled to and transmitted through the fiber. The position of the fiber is incrementally adjusted relative to the laser either manually or using a machine until the light transmitted through the fiber reaches a maximum, at which time, the optical fiber is permanently bonded to the same substrate that the laser was previously bonded to.
An optoelectronic photodiode, such as a PIN or APD photodiode, may similarly be coupled to an optical fiber through "active alignment" by bonding the photodiode to a substrate and positioning the end of the optical fiber that is to be coupled to the photodiode in proximity to the light-receiving surface of the photodiode. Light is then radiated through the opposing end of the optical fiber using a light source and the position of the fiber is incrementally adjusted relative the photodiode until the photodiode's electrical response reaches a maximum, wherein the optical fiber is then bonded to the substrate supporting the photodiode.
Alternatively, such "active alignment" of an optoelectronic device (e.g., laser or photodiode) to an optical fiber has been attempted by initially bonding the optical fiber to the substrate, moving the optoelectronic device into alignment by detecting the maximum optical radiation through the fiber, and then bonding the aligned optoelectronic device to the substrate supporting the fiber.
It is readily apparent that conventional active alignment techniques are inherently time consuming and require significant expertise and experience. Thus, with the increased demand for greater volumes of mass-produced optoelectronic modules, such time consuming assembly techniques become increasingly detrimental, thereby contributing to the relatively high manufacturing costs associated with conventional optoelectronic modules.
Indeed, contemporary assembly of conventional or hybrid microelectronic packages using conventional "pick-and-place" tools and surface mounting techniques have proven inefficient and insufficient for repeatedly mass producing optoelectronic modules. A number of micrometer-precision manufacturing tools and processes have also been developed for manufacturing optoelectronic modules, each having their own unique set of trade-offs for ease of use, cost, yield, extent of automation, upgradeabililty, flexibility, repeatability, accuracy, resolution, precision and propensity for bond liability after fixing. Such automated optoelectronic assembly equipment are typically high in cost due to the fact that they are usually custom or semi-custom built, large in size and weight, limited in part feeding ability and inadequate in resolution to perform single mode fiber alignment which requires alignment and bonding processes within tolerances of 1 .mu.m or less. As such, the optoelectronics industry has been required to rely heavily on labor-intensive manual assembly techniques for manufacturing high-performance optoelectronic modules.
Another problem associated with developing cost-effective techniques for assembling optoelectronic modules at the required high level of precision is achieving dimensional stability during bonding of the optoelectronic device and optical fiber to the substrate. Conventional bonding processes, such as laser welding and epoxy bonding, frequently result in residual stresses in the bonds that may cause undesirable creep and misalignment between the components of the optoelectronic module.
Solder alloys are widely used in the optoelectronics industry for bonding optoelectronic devices to submounts inside optoelectronic package housings. Some of the more common submount materials include aluminum nitride, beryllium oxide, beryllium-copper alloy, copper, copper-tungsten alloy, diamond, molybdenum and silicon. Because most optoelectronic devices are made from Group III-V (e.g., GaAs, InP, etc.) and their ternary and quaternary alloys (e.g., GalnAs, GalnAsP, GaInAsP, etc.), the submount materials upon which the optoelectronic devices are bonded generally have dissimilar mechanical and thermal properties. In environments where temperature cycling is expected (e.g., commercial aerospace platforms and outdoor fiber-optic cable systems), high thermal stresses and creep strains may build up in the solder joints, potentially leading to premature joint failure and shortened operating life.
During conventional optoelectronic device die-bonding processes, solder is heated to approximately 300.degree. C. to melt it, enabling the solder to wet the submount and optoelectronic device metalization layers. When cooled, the solder re-solidifies to form a metallurgical bond between the device and submount. It is known that a die-bonded optoelectronic device-to-submount assembled in this manner is mechanically and electrically robust and reliable if the coefficients of thermal expansion (CTE) are matched to within a few parts-per-million per degree Celsius, and if proper metallurgy is selected at the solder joint interfaces. Upon thermal cycling, however, both the magnitude and spatial relationship of some thermal stress/strain components in the solder joint change, which, in some cases, causes premature failure in the device or induce undesirable mechanical movement between the device and submount. Today, die-bonding accuracies achieved by conventional die bonding methods are in the 50 .mu.m range due to the lack of special machine vision and/or active alignment die-bonding process technology.
More recently, a new optoelectronic device bonding technique known as "self-alignment" based upon solder bump flip-chip technology has been employed to reduce die bonding accuracies from tens of micrometers toward a few micrometers. In this "self-alignment" process, small (approximately 75 .mu.m diameter) solder bumps are placed around the periphery of the optoelectronic device. These solder bumps serve to "self-align" the device (i.e., through surface tension) as the solder is heated to a molten state and during reflow of the solder. When coupling light between optical fibers or waveguides and optoelectronic devices, the self-alignment process eliminates the need for actively adjusting the position of the device relative the fiber or waveguide when the solder is molten. This self-alignment process, however, has only been successfully used to assemble optoelectronic modules where the optomechanical tolerances are fairly loose (e.g., approximately 10 .mu.m) and has not yet been shown to be production-worthy in single mode optoelectronic circuits where a few micrometer bonding accuracy is considered too coarse, leaving the highly labor-intensive and time-consuming active alignment method as the only production-worthy alternative.